As the radio spectrum becomes more crowded, the need for spectrally efficient radio technologies increases. One such technology is known as full-duplex radio that can transmit and receive at the same time and same frequency. Advantageously, full-duplex radio technology appears on the verge to be proven feasible for many commercial and military applications, a key technical challenge still remains that is termed self-interference cancellation (SIC). A full-duplex radio must have at least one radio transmit chain and one radio receive chain. The signal emitted out of the transmit chain is also picked up by the receive chain, creating self-interference.
The self-interference can be reduced by first increasing the attenuation between the transmit chain and the receive chain. This attenuation can be achieved by using various antenna technologies and even possibly using some radio blocker/absorber in between the transmit and receive antennas in some situations. This approach is also called passive cancellation in the literature.
The remaining self-interference has to be actively canceled by one or more SIC methods. For any SIC method, a cancellation waveform must be first generated based on a source signal from the transmit chain and then used for cancellation somewhere in the receive chain.
For radio interference cancellation, there are four stages which can be used in sequence for maximal benefit: passive interference suppression, all-analog interference cancellation, hybrid interference cancellation, and all-digital interference cancellation. Among the four stages, all-analog interference cancellation is the least mature.
Given the radio frequency (RF) nature of the interference, it is natural to think of an analog cancellation path between the transmitter and the receiver at the RF frontend. This is exactly what was proposed by M. Jain, J. I. Choi, T. M. Kim, D. Bharadia, S. Seth, K. Srinivasan, P. Levis, S. Katti, and P. Sinha, in their paper titled “Practical, real-time, full duplex wireless”, In Proc. Mobicom 2011; and J. G. McMichael and K. E. Kolodziej, in their paper titled “Optimal tuning of analog self-interference cancelers for full-duplex wireless communication”, 5th Annual Allerton Conference, October 2012, where a tunable analog circuit is used for interference cancellation. These analog methods will be referred to in this disclosure as all-analog where the cancellation path has analog input interface, analog filter and analog output interface as shown in FIG. 1. An advantage of all-analog passive circuits is that virtually no noise is introduced and the remaining interference can be further canceled at a later stage.
The Stanford method, as disclosed in S. Hong, J. Mehiman, and S. KattiPicasso's paper “Picasso: Flexible RF and Spectrum Slicing”, SIGCOMM'12, Helsinki, Finland, Aug. 13-17, 2012, can only handle all-pass interference channel. This all-pass condition fails completely if passive interference suppression has been applied (or if the transmit antenna and the receive antenna are far apart from each other). The passive interference suppression typically blocks the line-of-sight propagation between the antennas, which makes the interference channel highly frequency-selective. For intra-base station interference cancellation, passive interference suppression (via antenna nulling and other means) is desirable before any active interference cancellation method is used.
The MIT method, as disclosed in J. G. McMichael and K. E. Kolodziej' paper “Optimal tuning of analog self-interference, cancelers for full-duplex wireless communication,” 5th Annual Allerton Conference, October 2012, requires a joint tuning of one set of attenuators as well as another set of phase shifters. Even if the desired (complex) value of the attenuation of an attenuator and the phase of a phase-shifter is available, it is very difficult (if not impossible) to implement it accurately with the current analog technology. This is because the phase of a phase shifter is hard to control digitally and furthermore the phase of a phase shifter is highly coupled with its insertion loss (and hence highly coupled with the overall attenuation of a path comprising of the attenuator and the phase shifter). And furthermore, the residual interference from the MIT cancellation circuit is a highly nonlinear function of the joint set of the highly coupled tuning parameters phases and attenuations. It is extremely hard to find the optimal tuning even if the residual interference was not distorted by any unknown transfer functions (such as H_1(f) and H_2(f)). The cancellation results are all based on computer simulation assuming zero coupling between attenuations and phases. No hardware-based cancellation result was shown.
An alternative to all-analog is all-digital. There are well established theories for adaptive filters in the prior art that can be readily implemented in baseband digital signal processing (DSP) circuits. An all-digital cancellation path has digital input interface, digital filter and digital output interface. But this method works only if the interference, or residue interference after an initial cancellation, is not much stronger than the desired signal from a remote radio or otherwise the desired signal suffers from a large quantization noise. Furthermore, this method also suffers from the transmission noise. The interference caused by the noise originated from the transmit chain cannot be regenerated in the baseband for cancellation.
The alternative to all-analog and all-digital is hybrid. In order to preserve the desired (weak) signal in the receive chain, the strong interference should be canceled at the RF frontend of the receiver. For this purpose, several authors have proposed various forms of transmit beamforming based methods (see for example A. Sahai, G. Patel, and A. Sabharwal, “Pushing the limits of full-duplex: Design and real-time implementation”, Online at arXiv, 2011; T. Riihonen, S. Werner, and R. Wichman, “Mitigation of loopback self-interference in full-duplex MIMO relays,” IEEE Trans Signal Proc., Vol. 59, No. 12, December 2011; Y. Hua, “An overview of beamforming and power allocation for MIMO relays,” Proc of MILCOM 2010, pp. 99-104, San Jose, Calif., November 2010; and Y. Hua, P. Liang, Y. Ma, A. Cirik and Q. Gao, “A method for broadband full-duplex MIMO radio,” IEEE Signal Processing Letters, 2012) where the transmitters are prefiltered such that the waveform from a primary transmit chain and the waveform from a secondary (cancellation) chain cancels each other at the receiver's RF frontend. These methods will be referred to as hybrid-1 as shown in FIG. 1. The cancellation path is driven by a digital source waveform and also filtered digitally, but however the output of the cancellation path cancels the interference in an analog fashion at the RF frontend of the receiver. Compared to all-digital, the hybrid-1 reduces the burden of potential saturation of the receiver's frontend. But it still suffers from the transmission noise as for all-digital. Therefore, there is a need for a new hybrid radio interference cancellation.